Until relatively recently, the separation of plasma from whole blood, such as for the purpose of utilizing the plasma from a donor for transfusion to others or for some sort of treatment of the blood, has been carried out by extracting the whole blood and centrifuging it to separate the plasma. Such procedures are not only time consuming and cumbersome requiring large amounts of manual and mechanical handling, but they require a relatively large amount of expensive equipment.
Recently investigations have been conducted into so-called on-line membrane plasma separation techniques. In this type of separation, the blood is passed through the on-line separation equipment and immediately returned to the donor, the plasma being separated and, in the case of a donor, collected and stored, or alternatively in a case of a patient, treated, such as by sorption of a solute which it is desired to remove, and then returned to the patient. In addition to being much simpler than the centrifugal techniques of plasma separation in terms of the manual and mechanical handling of the blood, and lower expense, this technique when used for the treatment of the plasma has advantages over the conventional treatment of solute removal which incorporate treating agents in direct contact with whole blood in that in many instances the solute which it is desired to remove is concentrated in the plasma, making the technique a rapid and simple way to cleanse the blood. The most suitable treatment material can be chosen and used for the particular treatment desired without the need to consider the effect of the treatment material on the blood cells, since blood cell-treating agent interaction are eliminated, multiple types of treating agents may be employed, and it is easy to filter the treatment material from the plasma before reinfusing it into the patient, thus providing a treatment methodology that is safe in a wide variety of applications.
One method of carrying out such a plasma separation technique and an apparatus therefor are disclosed in U.S. Pat. No. 3,705,100 to Blatt, et al. A membrane arrangement is shown in which the pore sizes are 0.1 to 0.8 microns, and the blood is passed over the surface of the membrane at a flow rate of 2-50 feet per minute while a pressure differential across the membrane is maintained at from 1 to 15 psi. This method and apparatus are said to be effective in separating the plasma from the whole blood.
A similar method is disclosed in K. Ouchi et al, An Efficient, Specific and Blood Compatible Sorbent System for Hepatic Assist, Vol. XXIV Transactions American Society Artificial Internal Organs 246, 1978, in which a cellulous acetate filter was used to separate plasma from whole blood flowing through the hollow fiber membranes at a rate of 100 ml/min. and at pressure differentials across the membrane of 60,100 and 137 mm Hg., the separation at these pressures being 37.+-.2 ml/min., 34.+-.2 ml/min. and 32.+-.2 ml/min. respectively.
Recently issued U.S. Pat. No. 4,191,182 to Popovich et al also discloses a method and apparatus for continuous separation of plasma by so-called ultra-filtering. As with the above-described prior art, Popovich et al also operate at transmembrane pressures in the 50-700 mm Hg. range. While recognizing that transmembrane pressures which are too high will cause damage to the cellular components of the blood and that control of transmembrane pressure is necessary to avoid clogging of the filter, Popovich et al nevertheless prefer to operate in the 100-400 mm Hg. range of transmembrane pressures.